The overall goal of the proposed dissertation research project (R36) is to conduct a cross-sectional mixed methods study of sexual partner concurrency (i.e., overlapping sexual partnerships), an important factor in the epidemiology of HIV and other sexually transmitted infections (STIs), among dyads of high-risk, drug using female sex workers (FSWs) and their non-commercial, intimate male partners in the U.S.-Mexico border region. Substance abuse, sex work, and population mobility are features of the region, where a dynamic epidemic of HIV/STIs has emerged in high-risk populations including FSWs and injection drug users (IDUs). Although to date these HIV/STI epidemics remain concentrated, concurrency among FSW-intimate partner dyads could rapidly promote the bridging of HIV/STIs to the general population. Despite the vast literature on concurrency, little is known about the motivations for and contexts surrounding concurrency among marginalized, drug-using dyads in resource-poor settings in the Western Hemisphere. The proposed dissertation study seeks to characterize the social context of concurrency for high-risk dyads (Aim 1) and determine the gender-specific prevalence and correlates of concurrency among high-risk FSWs (Aim 2) and their intimate male partners (Aim 3) in the U.S.-Mexico border region. To meet these aims, we will implement a cross-sectional mixed methods design using qualitative and quantitative methods within Proyecto Parejas (R01-DA02772; PI: S. Strathdee), an existing cohort of 200 dyads of high-risk, drug-using FSWs and their intimate male partners in Tijuana and Cd. Juarez (N=400 subjects total). To meet Aim 1, we will purposively sample ~20 women and ~20 men (N=40) reporting recent (past year) concurrency in existing baseline quantitative surveys that measured concurrency using a UNAIDS-recommended sexual partner history module. We will conduct semi-structured interviews with this subsample, which will be distinct from data collection of the parent study, to qualitatively explore the meanings of and motivations for concurrency including social and environmental factors that may promote or discourage concurrency behaviors. We will then use emergent qualitative findings to develop a conceptual framework and specific hypotheses to guide quantitative analyses. To meet Aims 2 and 3, we will use existing quantitative baseline data from the entire cohort of 200 dyads (N=400). We will determine concurrency prevalence and apply multilevel (i.e., dyadic) logistic regression to identify individual- and relationship-level correlates of concurrency for FSWs (Aim 2) and their intimate male partners (Aim 3). Our dyadic, mixed methods study design will provide the first in-depth investigation of concurrency in a region characterized by prevalent drug use, commercial sex, and population mobility. It will also position the PI as a competitive new investigator in drug abuse research. Findings will be critical to the development of evidence-based, gender-specific interventions to reduce HIV/STI risk among drug-using FSWs and their intimate male partners.